Tamotsu Nakano

Measurements of LET-distribution, dose equivalent and quality factor with the RRMD-III on the Space Shuttle Missions STS-84, -89 and -91

Dosimetric measurements on the Space Shuttle Missions STS-84, -89 and -91 have been made by the real-time radiation monitoring device III (RRMD-III). Simultaneously, another dosimetry measurement was made by the Dosimetry Telescope (DOSTEL) on STS-84 and by the tissue-equivalent proportional counter (TEPC) on STS-91. First, the RRMD-III instrument is described in detail and its results summarized. Then, the results of DOSTEL and TEPC are compared with those of the RRMD-III. Also, the absorbed doses obtained by TLD (Mg2SiO4) and by RRMD-III on board STS-84 and -91 are compared.

LET distribution measurement with a new real-time radiation monitoring device-III onboard the Space Shuttle STS-84

Abstract A new type of Real-time Radiation Monitoring Device, RRMD-III, consisting of three double-sided silicon strip detectors (DSSDs) has been constructed and used onboard the Space Shuttle mission STS-84. The Space Shuttle cruised at an altitude of 300–400 km and an inclination angle of 51.6° for 221.3 h. RRMD-III succeeded in measuring the linear energy transfer (LET) distribution over the range of 0.2– 600 keV / μ m for 178 h . The obtained LET distribution of particles was investigated in detail by classifying it into galactic cosmic ray (GCR) particles and trapped protons in the South Atlantic Anomaly (SAA) region. The result shows that GCR particles contribute 60% to the total dose equivalent. The total absorbed dose rate during the mission was 0.516 mGy / day , the effective quality factor was 1.81 by ICRP-Pub.26, and the dose equivalent rate was 0.935 mSv / day . The average absorbed dose rates are 0.120 μGy / min for GCR particles and 4.80 μGy / min for trapped protons. The effective quality factors are 3.16 for GCR particles and 1.19 for trapped protons. RRMD-III data were also compared with the data of the tissue equivalent proportional counter (TEPC), proving that RRMD-III is a reliable device for deriving the true-LET distribution in real time for evaluating space radiation.

Analysis of deletion mutations of the rpsL gene in the yeast Saccharomyces cerevisiae detected after long-term flight on the Russian space station Mir

Using the yeast Saccharomyces cerevisiae on board the Russian space station Mir, we studied the effects of long-term space flight on mutation of the bacterial ribosomal protein L gene (rpsL) cloned in a yeast-Escherichia coli shuttle vector. The mutation frequencies of the cloned rpsL gene on the Mir and the ground (control) yeast samples were estimated by transformation of E. coli with the plasmid DNAs recovered from yeast and by assessment of the conversion of the rpsL wild-type phenotype (Sm(S)) to its mutant phenotype (Sm(R)). After a 40-day space flight, some part of space samples gave mutation frequencies two to three times higher than those of the ground samples. Nucleotide sequence analysis showed no apparent difference in point mutation rates between the space and the ground mutant samples. However, the greater part of the Mir mutant samples were found to have a total or large deletion in the rpsL sequence, suggesting that space radiation containing high-linear energy transfer (LET) might have caused deletion-type mutations.

The effects of microgravity on induced mutation in Escherichia coli and Saccharomyces cerevisiae.

We examined whether microgravity influences the induced-mutation frequencies through in vivo experiments during space flight aboard the space shuttle Discovery (STS-91). We prepared dried samples of repair-deficient strains and parental strains of Escherichia (E.) coli and Saccharomyces (S.) cerevisiae given DNA damage treatment. After culture in space, we measured the induced-mutation frequencies and SOS-responses under microgravity. The experimental findings indicate that almost the same induced-mutation frequencies and SOS-responses of space samples were observed in both strains compared with the ground control samples. It is suggested that microgravity might not influence induced-mutation frequencies and SOS-responses at the stages of DNA replication and/or DNA repair. In addition, we developed a new experimental apparatus for space experiments to culture and freeze stocks of E. coli and S. cerevisiae cells.

LET distributions from CR-39 plates on Space Shuttle missions STS-84 and STS-91 and a comparison of the results of the CR-39 plates with those of RRMD-II and RRMD-III telescopes.

The LET distributions during the Space Shuttle missions STS-84 (altitude 270-412 km, average 375 km; inclination angle, 51.6 degrees) and STS-91 (altitude 328-397 km, average 373 km; inclination angle, 51.6 degrees) were measured using CR-39 plastic nuclear track detectors. A correction for the dip-angle dependence of the track-formation sensitivity of the CR-39 plates was applied to the data analysis. The absorbed doses and the dose equivalents around RRMD Detector Units, estimated from the LET distributions in the LET region of 4-200 keV/micrometers, fluctuated with standard deviations of +/- 21% to +/- 35% in both flight experiments. The LET distributions obtained from the CR-39 plates agreed well with that obtained from RRMD-II in STS-91. However, the particle fluxes obtained from RRMD-III in STS-84 and STS-91 were two or three times higher than those obtained from RRMD-II and the CR-39 plates. It was concluded that the LET distributions obtained from RRMD-II and the CR-39 plates in the present flight experiments did not include the contribution of target-fragmented secondary heavy particles produced by low-LET particles, such as relativistic or semi-relativistic protons and helium ions, whereas RRMD-III was able to detect these secondary particles because of its low triggering level.

Radiation Dosimetry Measurements with Real Time Radiation Monitoring Device (RRMD)-II in Space Shuttle STS-79

The real-time measurement of radiation environment was made with an improved real-time radiation monitoring device (RRMD)-II onboard Space Shuttle STS-79 (S/MM#4: 4th Shuttle MIR Mission, at an inclination angle of 51.6 degrees and an altitude of 250-400km) for 199 h during 17-25 September, 1996. The observation of the detector covered the linear energy transfer (LET) range of 3.5-6000 keV/micrometer. The Shuttle orbital profile in this mission was equivalent to that of the currently planned Space Station, and provided an opportunity to investigate variations in count rate and dose equivalent rate depending on altitude, longitude, and latitude in detail. Particle count rate and dose equivalent rate were mapped geographically during the mission. Based on the map of count rate, an analysis was made by dividing whole region into three regions: South Atlantic Anomaly (SAA) region, high latitude region and other regions. The averaged absorbed dose rate during the mission was 39.3 microGy/day for a LET range of 3.5-6000 keV/micrometer. The corresponding average dose equivalent rates during the mission are estimated to be 293 microSv/day with quality factors from International Commission on Radiological Protection (ICRP)-Pub. 60 and 270 microSv/day with quality factors from ICRP-Pub. 26. The effective quality factors for ICRP-Pub. 60 and 26 are 7.45 and 6.88, respectively. From the present data for particles of LET > 3.5keV/micrometer, we conclude that the average dose equivalent rate is dominated by the contribution of galactic cosmic ray (GCR) particles. The dose-detector depth dependence was also investigated.

Dose equivalents inside the MIR Space Station measured by the combination of CR-39 plates and TLDs and their comparison with those on Space Shuttle STS-79, -84 and -91 missions.

In 1997, four dosimeter packages, each of which contains two CR-39 plates and 18 TLDs (Mg2SiO4:Tb), were placed inside the MIR Space Station and flew on an orbit with an inclination angle of 51.6 degrees and an altitude of approximately 400 km for 40 days. We estimated the absorbed doses, dose equivalents and effective quality factors during the flight by combining CR-39 data and TLD data. We then compared these results to those obtained with the same analysis method from the dosimeter packages on board Space Shuttle missions STS-79, -84 and -91 that flew along the same orbit. Finally, the differences between our results and those obtained by another group using passive dosimeters on the MIR are discussed.

Differentiation of Dictyostelium discoideum vegetative cells into spores during earth orbit in space

We reported previously that emerged amoebae of Dictyostelium (D.) discoideum grew, aggregated and differentiated to fruiting bodies with normal morphology in space. Here, we investigated the effects of space radiation and/or microgravity on the number, viability, kinetics of germination, growth rate and mutation frequency of spores formed in space in a radiation-sensitive strain, gamma s13, and the parental strain, NC4. In gamma s13, there were hardly spores in the fruiting bodies formed in space. In NC4, we found a decrease in the number of spores, a delay in germination of the spores and delayed start of cell growth of the spores formed in space when compared to the ground control. However, the mutation frequency of the NC4 spores formed in space was similar to that of the ground control. We conclude that the depression of spore formation might be induced by microgravity and/or space radiation through the depression of some stage(s) of DNA repair during cell differentiation in the slime mold.

Detection of DNA damages and repair in human culture cells with simulated space radiation

DNA damages and its repair of cultured WI38 human fibroblast cells and T98G human glioblastoma cells were studied by exposing to carbon ion beams of HIMAC accelerator. The exposed cells were incubated at 37 degrees C for appropriate intervals and the damages were analyzed by alkaline comet assay and quantitative RT-PCR with p53 mRNA. Highly inhomogeneous DNA damages were observed among the electrophoretic cell images of the comet assay. The degree of the damages was analyzed semi-quantitatively by using the Comet Index. The damaged fraction of WI38 cells was 85% immediately after 4 Gy (100keV/micrometer) irradiation and decreased to 50% after 120 min. incubation indicating a repair of cell DNA. Time dependent p53 gene expression as also analyzed by the quantitative RT-PCR method.

Alkylating agent (MNU)-induced mutation in space environment

In recent years, some contradictory data about the effects of microgravity on radiation-induced biological responses in space experiments have been reported. We prepared a damaged template DNA produced with an alkylating agent (N-methyl-N-nitroso urea; MNU) to measure incorrect base-incorporation during DNA replication in microgravity. We examined whether mutation frequency is affected by microgravity during DNA replication for a DNA template damaged by an alkylating agent. Using an in vitro enzymatic reaction system, DNA synthesis by Taq polymerase or polymerase III was done during a US space shuttle mission (Discovery, STS-91). After the flight, DNA replication and mutation frequencies were measured. We found that there was almost no effect of microgravity on DNA replication and mutation frequency. It is suggested that microgravity might not affect at the stage of substrate incorporation in induced-mutation frequency.